In recent years, there is an increasingly growing demand of reliability for semiconductor devices for electric power, and in particular, there is a demand to improve durability and reliability of a bonding portion between a semiconductor element for electric power and a circuit board having a large difference in thermal expansion coefficient therebetween. Heretofore, semiconductor elements for electric power whose base materials are silicon (Si) and gallium arsenide (GaAs) have been often used, and their operation temperatures are from 100° C. to 125° C. When these elements are bonded to circuit boards, soldering materials have been often used.
On the other hand, from the aspect of saving energy, semiconductor elements for electric power whose base materials are silicon carbide (SiC) and gallium nitride (GaN) have been under active development as next-generation devices.
They operate with a large current and have operation temperatures of 175° C. or more, and it is said that the temperature will become 300° C. in future. On that occasion, in the case where bonding is made by melting a bonding material itself, such as a solder, it is required to make bonding at a temperature higher than a heat resistant temperature of the bonding portion, so that, not just the number of choices for a bonding material itself is limited, there are restrictions to prevent degradation of the bonding target.
In this respect, attention is paid to a bonding material which is a so-called sinterable metal or metal paste and in which nano or micron-sized metal particles and an organic solvent are mixed together (see, for example, Patent Document 1). According to such a bonding material, when the organic component covering the surfaces of the metal particles is decomposed by heat, the metal particles are sintered with each other to form a bonding portion and the heat resistant temperature after sintering (bonding) becomes a temperature (for example, 960° C. in the case of silver) that is nearly equal to the melting point of the solid metal. Though depending on what the organic component is, the organic solvent is decomposed at about 200 to 300° C., so that it is possible to make bonding at a temperature where the bonding target is not degraded, and to achieve enhanced heat resistance after bonding.
Meanwhile, as described in Patent Document 1, in comparison with the solid metal, the metal-sintered material has a lower elastic modulus due to having voids. Notwithstanding, it is still higher in elastic modulus in comparison with the conventional solders, etc., so that its stress-relaxation ability in heat cycle decreases and thus, it is difficult to keep the bonding strength over a long period of time. In this respect, there is proposed a technology in which particles having lubricating ability or fillers of a resin being lower in elastic modulus than the metal particles are mixed in the bonding material to thereby relax stress (see, for example, Patent Document 2 or 3).